This invention relates to the use of relays as switching devices in audio equipment and primarily in conjunction with amplifiers for electric guitars which are used to provide muting of unwanted noise caused by the switching action of relays in high gain audio circuitry.
As amplifiers for electric guitars evolved in complexity, the types and methods of switching, such as, but not limited to different channels and different modes of operation, grew in importance. Several types of switching devices have been used and each has its virtues and drawbacks.
The field effect transistor (FET) is suitable for switching low voltage signals, however the signal voltages present at many points in a modem vacuum tube guitar amplifier exceed the capability of most FETs.
The light-dependent-resistor (LDR) is the most widely used device for switching in guitar amplifiers. By combining an LED light source and a photosensitive resistor in a light-tight package, the LDR is not so limited in signal voltage which it can handle but it has other serious drawback of its own. First, the LDR is an expensive device, presently costing in excess of one dollar even in very large quantities. Like the FET, the switching function of an LDR is limited to single-pole single-throw action so a great number of these devices (two dozen or more) may be required in a modem multi-function amplifier. Of even greater importance is the fact that the LDR is an imperfect on-off switch going neither fully ON nor fully OFF. Not only do the ON and OFF resistances vary greatly from device to device, but the time required to change from the on state to the off state also varies greatly. Furthermore, the resistance of a given LDR is affected by temperature. LDRs are also sensitive to assembly processing and can easily be damaged by heat during the solder operation or by moisture while solder flux is being washed away.
Relays, simple electro-mechanical devices, avoid all of the aforementioned problems while typically providing a double-pole double-throw switching operation for less than the cost of a single LDR. The ON resistance of a relay is a consistent essentially zero ohms; the off resistance is always essentially infinite. Relays are easy to handle, assemble and test but their one major liability has limited their use in guitar amplifiers, this being the fact that they provide a xe2x80x9cpopxe2x80x9d noise. Because a relay breaks and then remakes circuit continuity instantaneously, a very audible popping noise is typically generated. In circuits where substantial gain occurs after a relay is operated, the popping sound can easily be amplified to wildly unacceptable levels of loudness. In many applications, the brief period of time during which the relay is out-of-contact and in motion, a momentary volume surge may occur as well.
In accordance with the present invention, the above described problem of xe2x80x9cpopxe2x80x9d is substantially eliminated from the amplifier output and the use of relays is enabled throughout a guitar amplifier. This is accomplished by providing an FET mute circuit to shunt the audio circuit and conceal the popping noise caused by the relays. Such a mute circuit must be triggered to occur exactly in synchronism with the relay switching in order to be effective. To this end, the relays themselves are used as triggers for the mute circuit. All that is required is to insure that in every possible switching operation at least one relay is switching off.
An electro-magnet is used in a typical relay to actuate a mechanical switch mechanism. When current is removed from the electromagnetic coil, an inductive pulse is generated. In the present invention, the voltage momentarily generated by the collapsing magnetic field of the coil is used to trigger a bipolar switching transistor which then drives an FET arranged to shunt or short circuit the guitar signal. Such an arrangement may be used in one or more locations in the amplifier circuit. Because the action of the bipolar and field-effect transistors is so nearly instantaneous, the muting effect occurs sufficiently rapidly to blank out the unwanted popping noise.
A time constant circuit is preferably utilized to maintain muting action for any desired duration of time. In some amplifiers including a reverberation circuit, a duration time of two or three seconds may be useful to mute reverberation decay of the previous mode from overlapping with the mode instantly selected. For normal shunting of popping noise, the time constant may be adjusted to a fraction of a second, sufficiently long to mute the unwanted noise but not so long as to become a noticeable xe2x80x9cdrop outxe2x80x9d of the musician. Because the duration of the shunt is determined by the component values of an out-board time-constant circuit, this duration is easily adjustable and there is no unpredictable variation from one FET to another.